As is well known, as a type of steam turbine, there is a steam turbine provided with a casing, a shaft body rotatably provided inside of the casing, a plurality of turbine vanes fixedly disposed at an inner peripheral portion of the casing, and a plurality of turbine blades radially provided at the shaft body at the downstream sides of the plurality of turbine vanes. In the case of an impulse turbine among such steam turbines, the pressure energy of steam is converted into velocity energy by the turbine vane and the velocity energy is converted into rotational energy (mechanical energy) by the turbine blade. Further, in the case of a reaction turbine, pressure energy is also converted into velocity energy by the turbine blade and the velocity energy is converted into rotational energy (mechanical energy) by a reaction force generated by the ejected steam.
In this type of steam turbine, a gap is formed between a tip portion of the turbine blade and a casing that surrounds the turbine blade, thereby forming a flow path of steam, and a gap is also formed between a tip portion of the turbine vane and the shaft body. Leakage flow (leak steam) flows toward the downstream side from the upstream side of main flow in these gaps. However, if the leakage flow joins the main flow at the downstream side of the main flow, the flow of the main flow is disturbed, whereby loss (hereinafter referred to as “mixing loss”) is generated, and thus turbine efficiency is reduced.
In PTL 1 below, there is proposed a configuration in which a guide plate guiding leakage flow is mounted on a shroud at the outlet side of a flow path of the leakage flow among the above-described gaps, thereby making the direction of the leakage flow conform to the direction of main flow flowing out from the turbine blade. By such a configuration, a disturbance of the main flow that is generated at the time of joining of the leakage flow and the main flow is suppressed, and thus the mixing loss is reduced.